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HHMathewChan
HHMathewChan

Posted on • Originally published at rebirthwithcode.tech

Python challenge: Leetcode 3. Longest Substring Without Repeating Characters

Question

  • Given a string s, find the length of the longest substring without repeating characters.

Example

  • Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

  • Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

  • Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

Constraints:

  • 0 <= s.length <= 5 * 104
  • s consists of English letters, digits, symbols and spaces.

My attempt

Attempt 1 (fail at last two case, exceed time limit)

  • algorithm
>Find the length of the longest substring without repeating characters.
initialise the length_of_s equals to length of s
initialise the length_list_of_sub_string to the empty string

define a find_and_record_substring program:
    initialise seen_string as an empty string
    initialise seen_dictionary as an empty dictionary
    initialise the length equals to 0
    for each character in s:
       if the charcter do not found in seen_string:
           add the character to seen_string
        otherwise:
            initialise the length equals to length of seen_string
            update seen_dictionary with length as key, seen_string as value
            set seen_string to empty string
            add the character to seen_string
    return the maximum of the key of the dictionary

>>find and record each substring without repeating characters in the string s

repeat the find_and_record_substring program for the length of s times:
    add the result of find_and_record_substring program to length_list_of_sub_string
    set s to a s that remove the first character

>>return the longest substring without repeating characters
return longest length equals to the maximum of length_list_of_sub_string
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  • code
class Solution:  
    def lengthOfLongestSubstring(self, s: str) -> int:  
        length_of_s = len(s)  
        length_list_of_sub_string = []  
        # program to find each substring without repeating characters in the string
        def find_and_record_substring(str1: str):  
            seen_string = ""  
            seen_dict = {}  
            for char in str1:  
                if char not in seen_string:  
                    seen_string += char  
                # if char found in seen_string
                else:
                    # add the current seen_string to dict first  
                    length_of_seen_string = len(seen_string)  
                    seen_dict.update({length_of_seen_string: seen_string})
                    # clear the seen_string and continue to build seen_string  
                    seen_string = ""  
                    seen_string += char
            # should add the current seen_string to dict
            length_of_seen_string = len(seen_string)  
            seen_dict.update({length_of_seen_string: seen_string})  
            longest_length = max(seen_dict.keys())  
            return longest_length  
        if s == "":  
            return 0  
        while length_of_s != 0:  
            length_list_of_sub_string.append(find_and_record_substring(s))
            # remove the first charcter of s  
            s = s[1::]
            # update the length of s  
            length_of_s = len(s)  
        return max(length_list_of_sub_string)
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Attempt 2 (success)

  • basically the same way as the above , but add a function to find the upper limit of a substring that is equal to the number of unique character in the string s.
  • and when any substring reach the upper limit, the function will not go any forward and return the substring length instead
  • code
class Solution:  
    def lengthOfLongestSubstring(self, s: str) -> int:  
        length_of_s = len(s)  
        length_list_of_sub_string = []  
        temp_max = 0  

        def longest(str1):  
            set1 = set(str1)  
            return len(set1)  

        max_limit = longest(s)  

        def find_and_record_substring(str1: str):  
            seen_string = ""  
            seen_dict = {}  
            for char in str1:  
                if char not in seen_string:  
                    seen_string += char  
                else:  
                    length_of_seen_string = len(seen_string)  
                    seen_dict.update({length_of_seen_string: seen_string})  
                    seen_string = ""  
                    seen_string += char  
            length_of_seen_string = len(seen_string)  
            seen_dict.update({length_of_seen_string: seen_string})  
            longest_length = max(seen_dict.keys())  
            return longest_length  

        if s == "":  
            return 0  
        while length_of_s != 0 and temp_max < length_of_s and temp_max < max_limit:  
            length_list_of_sub_string.append(find_and_record_substring(s))  
            s = s[1::]  
            length_of_s = len(s)  
            temp_max = max(length_list_of_sub_string)  
        return max(length_list_of_sub_string)
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Other solution

def lengthOfLongestSubstring(s: str) -> int:
    # Base condition
    if s == "":
        return 0
    # Starting index of window
    start = 0
    # Ending index of window
    end = 0
    # Maximum length of substring without repeating characters
    maxLength = 0
    # Set to store unique characters
    unique_characters = set()
    # Loop for each character in the string
    while end < len(s):
        if s[end] not in unique_characters:
            unique_characters.add(s[end])
            end += 1
            maxLength = max(maxLength, len(unique_characters))
        else:
            unique_characters.remove(s[start])
            start += 1
    return maxLength
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My reflection

  • so I learn a new algorithm today called Sliding Window Algorithm.

Credit

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